I think the RA and Dec of a celestial object should change (ever so slightly) with the Earth's position in its orbit. So do we ignore this in general?

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    $\begingroup$ What do you mean "the celestial sphere revolves" The motion of the "celestial sphere" is entirely the result of the motions of the Earth. Of course individual stars might move relative to the sphere, but these motions are uncoordinated. $\endgroup$
    – James K
    Commented May 26, 2023 at 6:16
  • $\begingroup$ RA & Dec are zero at the equinox, but the equinox direction changes slowly, see en.wikipedia.org/wiki/… Is that what you're asking about? $\endgroup$
    – PM 2Ring
    Commented May 26, 2023 at 6:40
  • $\begingroup$ @PM 2Ring Another way of putting my question is, Is the position of the earth w.r.t the sun gonna affect the RA and Dec? $\endgroup$ Commented May 26, 2023 at 6:47
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    $\begingroup$ @AmbicaGovind Are you thinking of parallax? $\endgroup$
    – Sten
    Commented May 26, 2023 at 7:11
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    $\begingroup$ @AmbicaGovind Parallax does change an object's sky coordinates (very slightly)! I think people (including myself) just didn't understand the question at first. $\endgroup$
    – Sten
    Commented May 26, 2023 at 7:38

1 Answer 1


This is called "parallax". The observed position of a star (as measured by RA and Dec) does vary ever so slightly.

For example, Proxima centauri, the closest star has a parallax of 0.77 arcseconds. For context, there are 3600 arcseconds in one degree. The moon is about 1800 arcseconds across.

The parallax can only be measured by very careful telescopic observations.

Other stars are further away, and have even smaller parallax.

The underlying RA and Dec grid stays fixed, but stars (in particular nearby stars) move relative to it.


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